The present invention relates to an actuator comprising a magnetic shape memory alloy member such as a spring, etc., which can be driven in a relatively small magnetic field, thereby giving large stroke and output force.
In the technical fields of robots, machine tools, automobiles, etc. utilizing electromagnetic motors, driving systems are required to have reduced weight. However, because the output densities of the electromagnetic motors depend on the weight of motors, there are limitations in the reduction of weight in the actuators using electromagnetic motors. Accordingly, demand is mounting on actuators providing large outputs with reduced size and weight.
Conditions required for the actuators are that a movable member is displaced to a desired position when driven, and returns to an original position without fail while idling, and that a large output is obtained to drive the movable member even though there is a large load. In order that the movable member returns to an original position without fail while idling, a spring should be used as a biasing member for the movable member. However, when the spring has a large repulsive force, a large force is required to drive the movable member against the spring force. Thus required is a spring deformable with a small force.
The shape memory alloys are attracting much attention particularly as actuator materials, because they can give as large displacement (shape recovery strain) as up to about 5%. The shape memory alloys plastically deformed at a certain temperature regain original shapes when heated to temperatures equal to or higher than their transformation temperatures. Thus, when an alloy is caused to memorize a certain shape by a heat treatment with its shape constrained in an austenite phase, a high-temperature phase, it can regain original shape due to a reverse transformation mechanism by heating, even after deformation in a martensite phase, low temperature phase. This phenomenon is utilized for actuators. However, a shape memory phenomenon by temperature change requires heating and cooling, and heat diffusion particularly at the time of cooling is rate-determining, resulting in slow response in the temperature control.
Accordingly, an actuator utilizing a magnetic shape memory alloy having a phase transformation structure (twin structure) was proposed (JP 11-509368 A and JP 2001-525159 A). The driving of this actuator utilizes deformation caused by the reorientation of martensitic unit cells (magnetization vectors in the cells) in the direction of a constant magnetic field, which is applied in a martensite state.
In thin ribbons (JP 11-269611 A) and thin films (JP 2001-329347 A) of magnetic shape memory alloys such as Fexe2x80x94Pd alloys, etc. having excellent workability, strain occurs to some extent by a relatively small, constant magnetic field of 1 to 2 kOe, but an extremely large, constant magnetic field is needed to cause strain (deformation) in bulk members such as springs. For instance, a magnetic field of 400 kOe is needed to deform bulk members of Fexe2x80x94Nixe2x80x94Coxe2x80x94Ti alloys, and springs of Fexe2x80x94Pd alloys are not deformed in a magnetic field of up to 20 kOe. Accordingly, actuators comprising magnetic shape memory alloy members giving large stroke and load in a relatively small magnetic field are desired.
Accordingly, an object of the present invention is to provide a precisely controllable actuator generating large displacement and force with good response when a relatively small magnetic field is applied.
As a result of intense research in view of the above object, the inventors have found that because a stress generating when a magnetic field is applied to a magnetic shape memory alloy having a twin structure is proportional to the gradient of the magnetic field applied, and because this stress causes deformation due to the reorientation of martensitic unit cells, an actuator can be driven by deposing its magnetic shape memory alloy in a slanting magnetic field. The present invention has been completed based on this finding.
Thus, the actuator of the present invention comprises a magnetic, resilient, shape memory member formed by a substance having a twin structure, and a magnetic field generator, at least part of the magnetic, resilient, shape memory member being disposed in a slanting magnetic field generated from the magnetic field generator such that the twin structure is reoriented by the magnetic field, whereby the shape memory resilient member is driven.
The actuator is driven with the twin structure reoriented by a stress generating in the gradient of a magnetic field applied. The magnetic, resilient, shape memory member is located preferably at a position at which the magnetic field has the largest gradient. Because the stress applied to the magnetic, resilient, shape memory member is maximum at a position at which the magnetic field has the largest gradient, the reorientation of the twin structure of the magnetic, resilient, shape memory member can be induced even in a low magnetic field.
The magnetic, resilient, shape memory member of the actuator may be a coil spring or a plate spring. The magnetic field generated from the magnetic field generator is preferably 20 kOe or less, and the substance having a twin structure is preferably an Fexe2x80x94Pd alloy from the aspect of workability. Accordingly, with the magnetic, resilient, shape memory member disposed in the gradient of the magnetic field, the coil spring or the plate spring can be displaced even in a low magnetic field of 20 kOe or less.
The actuator is driven preferably around the martensitic transformation-starting temperature Ms of the substance having a twin structure. With magnetic field generator disposed at both ends of the magnetic, resilient, shape memory member, the magnetic field can have a larger gradient.